Problem with Java and

1
Problem with Java and

• how it causes a problem for DJ

2
Problem with Java and DJ

• What is coming is not about a problem of DJ but
  about a problem with Java the lack of
  parameterized classes.
• The lack of parameterized classes forces the use
  of class Object which, as the mother of all
  classes, is too well connected.
• This leads to unnecessary traversals and
  traversal graphs that are too big.

3
DJ Traversals are opportunistic

• If there is a possibility that an object o may be
  on a path to success, o is visited.
• The decision whether there is a possibility is
  made based on whether there is a path in the
  class graph.
• The presence of class Object in a path in the
  class graph drastically increases the
  possibilities.

4
BList
A

X
B
Vector

Object
A
X
B
5
Traversal Strategy
find all persons waiting at any bus stop on a bus
route
from BusRoute through BusStop to Person
busStops
BusRoute
BusStopList
buses
0..
BusStop
BusList
waiting
0..
passengers
Bus
PersonList
Person
0..
6
Traversal Strategy
find all persons waiting at any bus stop on a bus
route
from BusRoute through BusStop to Person
busStops
BusRoute
BusStopList
Vector
buses
0..
BusStop
Object
BusList
waiting
0..
passengers
Bus
PersonList
Person
0..
7
Traversal Strategy
find all persons waiting at any bus stop on a bus
route
from BusRoute through BusStop to Person
busStops
BusRoute
BusStopList
Vector
buses
0..
BusStop
Object
BusList
waiting
passengers
Bus
PersonList
visit all Bus-objects and stop at them.
Person
0..
8
ObjectGraph Traversal
Bus17Bus
Vector
Route1BusRoute
buses
Bus16Bus
busStops
Vector
Bus15Bus
passengers
CentralSquareBusStop
waiting
PersonList
PersonList
JoanPerson
PaulPerson
SeemaPerson
EricPerson
9
ObjectGraph Traversal
Bus17Bus
Vector
Route1BusRoute
buses
Bus16Bus
busStops
Vector
Bus15Bus
passengers
CentralSquareBusStop
waiting
PersonList
PersonList
JoanPerson
PaulPerson
SeemaPerson
EricPerson
10
Unnecessary Traversal

• The traversal goes through the buses link
  although there will be no Bus-objects in that
  vector.
• But in Java, when we use a Vector-object, we
  cannot express in the class graph that we have
  only Bus-objects in that collection.

11
Lack of parameterized classes in Java makes DJ
harder to use

• Consider the traversal from A to B
• Lets assume that in the class graph between A
  and B there is a Java collection class. The
  intent is A List(B) which we cannot express in
  Java. Instead we have A Vector(Object). Object
  A B. Lets assume we also have a class XB.

12
Lack of parameterized classes in Java makes DJ
harder to use

• We have A Vector(Object). Object A B X.
  X B.
• If the vector contains an X object it will be
  traversed!!!

Vector

Object
A
X
B
13
No X-object is allowed to be in vector
A

X
B
Vector

Object
A
X
B
14
Moral of the story

• If the Collection objects contain only the
  objects advertised in the nice class graph of the
  application the traversal done by DJ will be
  correct. But unnecessary traversals still happen.
• However, if the Collection objects contain
  additional objects (like an X-object) they will
  be traversed accidentally.

15
Moral of the story

• Java should have parameterized classes.
• Workaround Use a JSR (Java Specification
  Request) 31 approach use a schema notation with
  parameterization to express class graph and
  generate Java code from schema. For traversal
  computation, the schema will be used.

16
Size of traversal graph

• DJ might create big traversal graphs when
  collection classes are involved. DJ will plan for
  all possibilities even though only a small subset
  will be realized during execution.
• To reduce the size of the traversal graph, you
  need to use bypassing. In the example from A
  bypassing A,X to B.